Measuring and Modeling Gravel Transport at Caspar Creek, CA, to Detect Changes in Sediment Supply, Storage, and Transport Efficiency

Abstract

We developed a technique for reconstructing annual gravel yields and generated a 55‐year record of gravel transport for the North Fork catchment of the Caspar Creek Experimental Watersheds in Northern California. The technique relies on field data collection including annual surveys of weir pond volumes and suspended sediment measurements, as well as an accounting for settling of suspended sediment and organic matter in the pond. We compared these annual yields to gravel yields predicted by the Wilcock two‐fraction bed load transport model, which we calibrated from measured values at Caspar Creek. We considered three velocity‐discharge relationships and found that values of hydraulic variables measured during storms produced the best fit between reconstructed and predicted annual gravel yields when years with large disturbances were excluded. We also compared predicted gravel transport rates to bed load transport rates measured from 1988 to 1995 with bed load pit samplers. We found that the calibrated model predictions agreed well with the field‐measured bed load transport rates. To investigate the role of supply and storage on gravel transport, we compared the reconstructed gravel yields to predicted gravel yields and found that increased occurrence of landslides and headcut erosion in the 1990s and early 2000s did not lead to an increase in gravel yields. Instead, input of large downed wood in the 1990s created storage space and decreased bed load delivery to the weir pond.

Effect of Sediment Supply on Cyclic Fluctuations of the Disequilibrium Ratio and Threshold Transport Discharge, Inferred From Bedload Transport Measurements Over 27 Years at the Swiss Erlenbach Stream

“…However, back‐calculated transport efficiencies e generally decrease for decreasing shear stresses smaller than about the reference shear stress (Abrahams & Gao, 2006; Eaton & Church, 2011; Recking, 2012; Shih & Diplas, 2018), and thus, e and E d will clearly differ in these flows. It is noted that in some other publications, the term sediment transport efficiency was used not in the strict sense of Bagnold's definition but in a more loose, relative sense, mostly in the way of comparing sediment (bedload) transport with hydraulic forcing for constant flow conditions or integrated over a flood event time scale (Piton & Recking, 2017; Rainato et al, 2017; Recking et al, 2012; Richardson et al, 2020). I have used here the disequilibrium ratio E d in a very similar way.…”

Fifty‐eight years and counting of watershed science at the Caspar Creek Experimental Watersheds in northern California

“…From 1962 to 1975, fixed‐stage samplers were used to collect stream samples on the rising limbs of the hydrographs at the NFC and SFC weirs. SSC from stream samples collected with fixed‐stage samplers were known to be overestimated, and different researchers developed approaches to address the bias (Keppeler, 2012; Lewis, 1998; Richardson et al, 2020). Pumping samplers were deployed at both weirs in 1975.…”

“…Rating equations are used to determine discharge from stage at each station except ARF and QUE. Rating equations were originally used to calculate discharge at the ARF site but were found to vary through the study (Richardson et al, 2020), so they were abandoned. ARF discharge can be estimated by subtracting discharge measured at the XYZ station from discharge measured at the NFC station for years when XYZ discharge data exist (Richardson et al, 2020).…”

“…Rating equations were originally used to calculate discharge at the ARF site but were found to vary through the study (Richardson et al, 2020), so they were abandoned. ARF discharge can be estimated by subtracting discharge measured at the XYZ station from discharge measured at the NFC station for years when XYZ discharge data exist (Richardson et al, 2020). A similar approach is used to estimate discharge at QUE because of a non‐stationary rating equation.…”

“…We expect that percent uncertainty decreases as the accumulated pond sediment volume increases. Richardson et al (2020) assumed that one standard deviation of the pond sediment volume was 20% of the annual pond sediment volume.…”